The present state of the art of surface and extremity coil building has many examples of efficient coils that resonate at two or even three frequencies, and that couple and decouple as RF pulses are applied. Until recently none of these coils utilized an adjustable coil geometry to vary the coil's electrical properties. W. G. Holcomb and J. C. Gore, "A Variable Frequency Birdcage Resonator For Imaging At High Field", Society of Magnetic Imaging, Tpm-C2, C3, disclose a variation of the standard cylindrical birdcage coil developed by C. E. Hayes, et al., "An Efficient, Highly Homogeneous Radiofrequency Coil For Whole-Body NMR Imaging at 1.5T", Journal of Magnetic Resonance 63, 622-628 (1965) that allows a radiologist to change the tuning capacitance and hence the resonant frequency of the coil by extending the length of the coil. This is accomplished by using concentric columns of electrically discrete copper tubing connected to opposite ends of the coil. Such a coil allows the radiologist to image samples of differing resonant frequencies without changing coils.
When a research radiologist desires to image a mouse, a rat, a rabbit, as well as a small dog, he would choose a coil providing the optimum filling factor for each. By doing so, the sample more completely fills the inside of the cylindrical coil, resulting in an improvement in the signal to noise ratio, and consequently an improvement in the images. Similarly, when a clinical radiologist images a human wrist, knee and head, he too would ideally choose different size coils for each if they were available. Often, however, the cost of either building or purchasing specialized coils is prohibitive so the radiologist must be satisfied with coils that are within easy access, and images that are second rate.